In recent years, FRP (Fiber Reinforced Plastics) cylinders have been used in various industrial fields. A filament winding method, as disclosed in Japanese Unexamined Patent Publication No. 2006-62355 in which carbon fibers are wound around a mandrel while being impregnated with a resin is known in the art as a method for producing an FRP cylinder. A prepreg method, as disclosed in Japanese Unexamined Patent Publication No. 2001-96635 in which a plurality of prepregs, each of which is made from carbon fibers impregnated with a thermosetting resin sheet, are wound and thermally cured to be formed as a plurality of FRP layers, is also known in the art as a method for producing an FRP cylinder.